1. Technical Field of the Invention
This invention relates generally to suspension components for two-wheeled vehicles, and more specifically to front steering and front spring/shock components.
2. Background Art
The vast majority of motorcycles (and full- and front-suspension bicycles) are equipped with front suspensions in which a pair of telescopic forks are coupled to the steering head of the motorcycle's frame by an upper triple clamp and a lower triple clamp. The triple clamps provide enough lateral offset that the forks clear the sides of the front tire. Trail is a measurement, on the ground, from a point projected through the steering axis to the center of the tire's contact patch directly below the axle, and determines the self-centering stability of the steering. The triple clamps are usually constructed to provide some measure of longitudinal offset, as well, to alter the trail. The forks are either of the conventional “right-side-up” or sliding-female configuration, or the “upside-down” or sliding-male configuration. In either case, a cylindrical tube or piston slides axially within a cylindrical cylinder.
In nearly all cases, both the suspension springs and the damping or shock absorbing components are disposed within one or both of the sliding tube assemblies. Unfortunately, because the substantial mass of the springs, dampers, oil, and other related suspension components is located a significant distance—generally in the neighborhood of 2.5 inches—from the axis of the steering head, the front end has an undesirably large moment of rotational inertia. In other words, the front end has a heavy “swing weight” which reduces both the “feel” and the responsiveness of the front end.
Alternative front end configurations have occasionally been seen, but all suffer from this same malady, and their inventors have been attempting to solve other problems, such as front end “dive” under hard braking, rather than reducing the moment of rotational inertia. Examples include the well-known BMW Telelever, the Britten front end, the Hossack front end, the RADD-Yamaha front end, and various hub-center systems such as that found on the Bimota Tesi.
None of these previous geometries places the spring or damper components coaxial to the steering head, and all suffer from having stylistic, aesthetic appearances which are dramatically different than the almost universally preferred conventional dual fork system. Furthermore, all are significantly more complex than the conventional dual fork system. The downside of these previous systems, such as increased mass, outweigh any benefit they may have offered on other fronts.
Some of the more advanced conventional telescopic forks provide external control knobs for adjusting some, but not all, of the hydraulic dampening characteristics of their internal dampening systems. For example, some forks have “clicker” adjusters for altering rebound dampening, compression dampening, and spring preload. There are other, more significant characteristics of conventional forks which are not externally adjustable, such as spring strength, oil quantity, oil viscosity, shim stacks, and so forth. Furthermore, existing telescopic fork front ends offer front ride height adjustment only by way of loosening the triple clamps and raising or lowering the upper fork tubes with respect to the triple clamps. This is a difficult, time consuming, and imprecise operation.
Fork flex, especially under braking, is a significant contributor to the stiction which is known to dramatically reduce the effectiveness and perceived quality of a motorcycle's front suspension.
As a motorcycle rider applies the front brake, the front forks are subjected to significant flexing force and torque in the direction of travel, as the rearward force on the front tire's contact patch presses rearward on the bottom of the forks at the axle, while the inertia of the motorcycle's mass presses forward on the top of the forks at the triple-clamps. Manufacturers battle this flex by using larger-diameter and thus stiffer fork tubes.
Fork flex, especially under braking, is a significant contributor to the stiction which is known to dramatically reduce the effectiveness and perceived quality of a motorcycle's front suspension. The manufacturer may battle this stiction by making even greater increases in the diameter and stiffness of the fork tubes, and by using expensively coated bushings, and so forth.
These engineering changes have an unfortunate side effect, which is exposed by the fact that motorcycles lean to the inside when cornering. In general, the faster a corner is taken, the farther over the motorcycle must lean. While leaned over, the axis of the fork suspension is not perpendicular to the ground, and yet the front tire's contact patch (which is at the center of the tire when riding straight, but is significantly off to the side of the tire when the bike is leaned over) remains parallel to the ground. Then, when the front tire encounters a bump in the road, the bump forces the tire in the vertical direction, perpendicular to the ground. But, because the forks are not oriented in that direction, the effect is that the force of the bump is applied to the forks somewhat laterally (in other words, radially or sideways), rather than axially with respect to the sliding ability of the fork tubes.
The forks' stiffness, which the engineer gave the fork tubes to counteract flex under braking, is now doing exactly the wrong thing with respect to the force of the bump—it is fighting the bump, rather than supplely allowing the front tire to track the road surface and remain in contact with the ground. Riders experience this as one form of front end chatter, especially when traversing an extended section of bumpy or rippled racetrack corner. The result is often a front end push which may end in a crash.
The motorcycle front suspension includes telescopic forks. Traditionally, bushings have been used to reduce friction between the inner fork tube and the outer fork tube. The upper end of the lower tube has a bushing, and the lower end of the upper tube has a bushing. Under non-axial loads, such as when braking, the mating surface of the lower fork tube is levered against the mating surface of the outer fork tube, significantly increasing the friction between the tubes. The shorter the distance of overlap—that is, the less the inner tube extends into the outer tube—the more pronounced this effect becomes, because the lever arm distance between the upper and lower bushings is reduced. And the greater the distance between the lowermost point of overlap and the ground (where the force is being applied), the greater this effect will be, because the longer the effective lever arm is.
Bearings provide lower friction than bushings. If the bushings were replaced with e.g. sets of ball bearings, the friction would be significantly reduced. However, the ball bearings concentrate the leverage force onto very small areas of the fork tubes, and can cause significant scoring and gouging of the fork tubes, especially if the fork tubes are made of a material which is not quite hard.
While a ball bearing concentrates its load at essentially a single point, a needle bearing spreads its load over a tremendously increased area, in essence a line the length of the bearing. However, while roller bearings offer the advantage of being able to travel in any direction, needle bearings are limited to traveling back and forth in a single direction.
Fork tubes are traditionally cylindrical, for a variety of advantageous reasons. Cylinders are relatively easy to machine to consistent and even tolerances. Two cylinders can be mated without any particular clocking (angular) requirements.
Trail is the distance, on the ground, from a point projected through the front axle on a line parallel with the steering axis, to a point directly below the front axle, or in other words, to the center of the contact patch. Trail directly impacts the steering stability of the motorcycle and its “return-to-center” force. Trail is affected by rake, which is the angle between vertical and the steering axis; steeper rake reduces trail. Trail is also affected by longitudinal fork offset, or the distance which the fork tubes are set in front of the frame's head tube; more offset decreases trail. Trail is also affected by axle offset; if the axle is coupled to the forks in front of their center, it increases trail.
The rider may wish to increase or decrease trail to, for example, change the steering feel or feedback, to improve steering quickness, or to eliminate a high-speed wobble, or to reduce a front end “push”. Often, riders will talk as though they are fixing these things by adjusting the ride height, which is generally discussed in terms of how far the forks extend up through the top triple clamp. However, decreasing front ride height by raising the forks farther through the triple clamps in reality steepens the rake (brings the forks closer to vertical), which, in turn, decreases trail (within the normal adjustment range). It is ultimately the change in trail which causes the effects which the rider attributes to his ride height adjustment.
Although adjusting trail can have very beneficial results, the other changes which go along with it in a conventional motorcycle may often—or even usually—outweigh or significantly counteract the benefits of the trail adjustment. For example, lowering ride height obviously puts the frame, engine cases, fairings, and other parts into closer proximity to the racetrack, often to an extent that cornering ability is actually reduced because hard parts of the motorcycle ground out on curbings or even the asphalt itself; it also changes the weight transfer bias under braking.
What is needed, then, is a system which has the aesthetic appeal and simplicity of the dual fork geometry, with a significantly reduced moment of rotational inertia. What is further needed is a system which offers reduced stiction.
What is also needed, then is an improved front fork which has suitably low lateral stiffness to better enable the front tire to track ground irregularities while leaned over cornering, without compromising its excellent longitudinal stiffness to resist flexing under hard braking. What is further desirable is such a fork which has adjustable lateral stiffness.
What is further needed is a mechanism which facilitates trail adjustments without adversely affecting other geometry of the motorcycle such as ride height and rake angle.